There are several types of liquid crystal displays (LCDs). For example, many watches utilize segment liquid crystal displays, in which each segment of liquid crystal material can be arranged into a template or pattern which can form any numeral (and virtually any letter). Each segment is then controlled to simply turn “on” or “off”; that is, transmit maximum light (appears brighter or white) or not (appears dim or black). Some advantages of segment LCDs include the small amount of space required by the display and the circuitry driving it, as well as low power consumption.
For more complex displays, such as those found in personal organizers or laptop computer monitors, a matrix LCD structure is normally utilized. A large number of small independent regions of liquid crystal material are positioned in a plane. Each of these regions is generally called a picture element or pixel. These pixels are arranged in rows and columns forming a matrix. Corresponding numbers of column and row electrodes are correlated with the rows and columns of pixels. An electric potential can therefore be applied to any pixel by the selection of appropriate row and column electrodes and a desired graphic can then be generated.
There are different types of matrix LCDs, such as active matrix LCDs and passive matrix LCDs. A passive matrix LCD uses a simple conductive grid to deliver current to the liquid crystals in the target area. Each row or column of the display has a single electrical circuit. The pixels are addressed one at a time by row and column addresses. As the number of pixels increases, however, this type of display becomes less feasible. Slow response time and poor contrast are typical of passive matrix LCDs.
In a conventional LCD driver circuit for driving a passive matrix LCD, passive devices such as resistive voltage dividers may be used to generate different bias voltages. This approach requires the LCD driver circuit to be active continuously which results high power consumption.